Add "copy-constructor" to prevent (incorrect) convert call when type …

…is the same (#36)

Before this change:
```
  julia> x = f = FixedDecimal{Int128,2}(3.25)
  >> FixedDecimal{Int128,2}(3.25)

  julia> @Btime x = FixedDecimal{Int128, 2}($f)
    7.272 μs (159 allocations: 2.73 KiB)
  >> FixedDecimal{Int128,2}(3.25)
```

After this change:
```
  julia> x = f = FixedDecimal{Int128,2}(3.25)
  >> FixedDecimal{Int128,2}(3.25)

  julia> @Btime x = FixedDecimal{Int128, 2}($f)
    1.508 ns (0 allocations: 0 bytes)
  >> FixedDecimal{Int128,2}(3.25)
```

src/FixedPointDecimals.jl

@@ -99,6 +99,9 @@ struct FixedDecimal{T <: Integer, f} <: Real
             _throw_storage_error(f, T, n)
         end
     end
+
+    # Copy constructor -- prevents unneeded work to convert between the same types
+    FixedDecimal{T,f}(x::FixedDecimal{T,f}) where {T<:Integer,f} = new{T,f}(x.i)
 end
 
 @noinline function _throw_storage_error(f, T, n)